1. Field of the Invention
The present invention relates to a semiconductor laser device that is used suitably as a light source for optical information processing, optical measurement or the like.
2. Description of Related Art
In recent years, because of their advantage of being compact and inexpensive, semiconductor laser devices have been used widely as a light source for optical information processing, optical measurement and the like.
Among those semiconductor laser devices, Fabry-Pxc3xa8rot (referred to as xe2x80x9cFPxe2x80x9d in the following)-type semiconductor laser devices using a FP resonator in which the semiconductor""s cleavage planes serve as mirrors are easy to manufacture and thus mass-produced as a light source for a pickup of optical disks. However, they have disadvantages in that their emission wavelength changes depending on temperature and that a laser oscillates at a plurality of longitudinal modes during a high-speed modulation.
In order to solve the above problems without compromising the compactness of the devices, a known semiconductor laser device adopted a technique in which a diffraction grating having a sharp wavelength selectivity is provided in the semiconductor resonator so as to perform light feedback. One of the semiconductor laser devices of this type is a distributed Bragg reflector (referred to as xe2x80x9cDBRxe2x80x9d in the following) type semiconductor laser device. The DBR type semiconductor laser device has a structure in which an active region for amplifying light and a DBR region provided with a diffraction grating are connected optically so that the light feedback is performed by utilizing a Bragg reflection in the DBR region.
The DBR type semiconductor laser device has the following advantages: (i) by applying electric current to the DBR region, a Bragg wavelength serving as the emission wavelength can be changed easily; (ii) since the active region and the DBR region are provided independently, the degree of design/production flexibility is high.
The following is a description of a conventional DBR type semiconductor laser device as a typical example. The structure of this DBR type semiconductor laser device described here is disclosed in IEEE JOURNAL OF QUANTUM ELECTRONICS VOL. 27, p. 1609.
FIG. 5 is a partially sectional perspective view showing a DBR type semiconductor laser device with a conventional structure. The DBR type semiconductor laser device is divided into three regions along its optical resonance direction. Numeral 201 denotes an active region, numeral 202 denotes a phase control region, and numeral 203 denotes a DBR region. Next, its layered structure will be described. On an n-type GaAs substrate (n-type substrate) 204, an n-type Al0.6Ga0.4As first cladding layer 205 and an active layer 206 are formed. The active layer 206 includes an undoped GaAs single quantum well and undoped AlxGa1xe2x88x92xAs (x=0.3 to 0.6) distributed refractive index (GRIN) layers that are arranged so as to sandwich the GaAs single quantum well from both external sides. An active layer 206a in the phase control region 202 and the DBR region 203 is disordered by Si ion implantation, thus providing a low-loss treatment for Bragg wavelength light. A rib-shaped n-type Al0.3Ga0.7As optical guiding layer 208 is formed on the active layer 206. The optical guiding layer 208 in the DBR region 203 is provided with a diffraction grating 208a. On the optical guiding layer 208, an n-type Al0.3Ga0.7As second cladding layer 211 and a p-type GaAs contact layer 212 are formed.
The contact layer 212 is arranged separately in the active region 201, the phase control region 202 and the DBR region 203 so that current can be applied independently to each region, and a p-electrode 213 further is provided thereon. The p-electrode 213 is formed immediately above the contact layer 212 in a rib region. In order to narrow the current applied from the p-electrode 213 to the rib region, an insulating layer 215 is provided immediately below the p-electrode 213 in a region other than the rib region. An n-electrode 214 is provided below the n-type substrate 204.
The following is a description of an operation of the conventional DBR type semiconductor laser device structured as above.
First, the current applied from the p-electrode 213 of the active region 201 is narrowed into the rib region by the insulating layer 215 and reaches the active layer 206, so that the active layer 206 in the rib region emits light. The rib region serves as a waveguide channel so as to propagate the emitted light.
When the semiconductor laser device is used as a light source for optical information processing or optical measurement, a single transverse mode is required. To meet this requirement, it is necessary to confine the guided light in the transverse direction effectively. In this DBR type semiconductor laser device with the conventional structure, since the rib-shaped optical guiding layer 208 is provided in the waveguide channel region, the effective refractive index in the waveguide channel region is lower than that in its outer regions, so that the guided light is confined in the transverse direction.
The waveguide structure of the semiconductor laser device that has been described here is a rib waveguide type. However, the operation is essentially the same in other refractive index waveguide structures, for example, a ridge waveguide type.
In this DBR type semiconductor laser device, an end face near the active region 201 and a DBR formed of the diffraction grating 208a in the DBR region 203 serve as two reflecting mirrors so as to form a resonator, so that the guided light is amplified in the active region 201 and emitted as a laser beam.
A plurality of longitudinal modes that satisfy a phase condition of the laser oscillation are present in the DBR type semiconductor laser device like in the FP type semiconductor laser device. Among these longitudinal modes, only the longitudinal mode having a wavelength closest to a Bragg wavelength of the DBR is Bragg-reflected mainly and satisfies an amplitude condition of the laser oscillation. Thus, the single longitudinal mode can be achieved. In this case, the Bragg wavelength xcexb is determined by an equation below.
xcexb=2Neqxcex9/qxe2x80x83xe2x80x83(1)
where Neq represents an equivalent refractive index of the DBR region 203, xcex9 represents a period of the diffraction grating, and q represents an order of the Bragg reflection. For instance, the Bragg wavelength is 850 nm in the DBR type semiconductor laser device of this conventional example.
The emission wavelength is controlled by current applied to the DBR region 203. When the current is applied, this changes the refractive index, that is, Neq in the DBR region. Therefore, the Bragg wavelength can be controlled according to Equation (1).
In this case, however, the emission wavelength only can change discontinuously at an interval of the longitudinal mode. This is because, among the longitudinal modes satisfying the phase condition as above, the one having a wavelength closest to the Bragg wavelength oscillates. In order to allow the emission wavelength to change continuously, it is necessary to control the phase of the guided light, thereby changing the wavelength of the longitudinal mode. For this purpose, the phase control region 202 for changing the phase of the guided light is provided inside the resonator. By applying current to the phase control region 202 so as to change the equivalent refractive index in this region, the phase of the guided light is controlled.
Thus, by setting the current applied to the phase control region 202 and the DBR region 203 appropriately, it is possible to allow the emission wavelength to change continuously.
The change in a refractive index N of a semiconductor by current application mainly is attributable to a plasma effect and a heat effect.
The plasma effect is caused by an applied carrier. The refractive index change xcex94Np due to the applied carrier xcex94ne is expressed by the following equation.
xcex94Np=xe2x88x92e2Nnexcex94ne/(2m*xcex5xcfx892)xe2x80x83xe2x80x83(2)
In Equation (2), e represents a unit charge, ne represents the number of carriers, m* represents an effective mass of electrons, xcex5 represents a dielectric constant, and xcfx89 represents an angular frequency of the light.
On the other hand, the heat effect is caused by power consumption during the current application. The refractive index change xcex94NT induced by current I is expressed by the following equations.
xcex94NT=+(∂N/∂T)ZTPDxe2x80x83xe2x80x83(3)
PD=IVj+I2Rsxe2x80x83xe2x80x83(4)
In Equations (3) and (4), T represents temperature, ZT represents heat resistance, PD represents power consumption, Vj represents forward voltage of p-n junction, and Rs represents series resistance. They are all values in the region where current is applied.
In a long-wavelength DBR type semiconductor laser device whose emission wavelength is in a 1.5 xcexcm band, for example, since the angular frequency xcfx89 of the emission wavelength is small, the refractive index changes mainly by the plasma effect.
On the other hand, in a DBR type semiconductor laser device whose emission wavelength is shorter than 900 nm such as the above-described conventional DBR type semiconductor laser device whose emission wavelength is in a 850 nm band, since the angular frequency xcfx89 is large, the plasma effect is small. Therefore, the refractive index changes mainly by the heat effect.
As described above, the DBR type semiconductor laser device whose emission wavelength is shorter than 900 nm can achieve the control of the emission wavelength by the heat effect of the applied current. Thus, in order to expand a tunable range of the emission wavelength, it is appropriate to raise the efficiency of heat generation by the applied current. However, the conventional DBR type semiconductor laser device needs to have a structure with excellent heat-radiating characteristics to avoid heat saturation and improve reliability. Accordingly, the conventional semiconductor laser device has had a problem that the efficiency of heat generation by the applied current is low, resulting in a limited tunable range of the emission wavelength.
It is an object of the present invention to provide a semiconductor laser device that can achieve both a high reliability and a wide tunable range of an emission wavelength even when the emission wavelength is shorter than 900 nm.
In order to achieve the above-mentioned object, a semiconductor laser device according to the present invention includes a first semiconductor layer having a diffraction grating in a region to be a Bragg reflection region, a current blocking layer that is provided above the first semiconductor layer and has a current application portion, a second semiconductor layer provided above the current blocking layer, and an electrode provided above the second semiconductor layer. The electrode is arranged at a region other than a region opposing the current application portion in the Bragg reflection region.
Preferably, the semiconductor laser device of the present invention further includes a contact layer above the second semiconductor layer and below the electrode. It is preferable that the contact layer is arranged at a region other than the region opposing the current application portion in the Bragg reflection region or that the contact layer has a portion that is arranged at the region opposing the current application portion, the portion being thinner than other portions in the Bragg reflection region.
Furthermore, the semiconductor laser device of the present invention also can have a structure in which a region including the region opposing the current application portion in the Bragg reflection region is provided with an insulating layer.
This structure elongates the path through which current applied to the Bragg reflection region passes, so that the series resistance in the distributed Bragg reflection region increases. Alternatively, the semiconductor laser device of the present invention may have a structure in which a waveguide channel region in the Bragg reflection region is provided with no electrode, and further is without a contact layer, or has a thin contact layer or an insulating layer. This lowers the heat-radiating characteristics with respect to heat generated in the waveguide channel region in the Bragg reflection region. As a result, the temperature increase owing to heat generation caused by the current applied to the Bragg reflection region becomes larger, thus achieving a wider tunable range of the emission wavelength. On the other hand, an active region associated with reliability has a structure with excellent heat-radiating characteristics as in the conventional semiconductor laser device. Consequently, both the high reliability and the wide tunable range of the emission wavelength can be achieved.
Moreover, the semiconductor laser device of the present invention preferably is provided with a phase control region, which has a similar structure to the Bragg reflection region of the semiconductor laser device of the present invention.
With this structure, the efficiency of heat generation by applied current improves also in the phase control region, so that the phase controllability can be raised. This reduces the applied current that is needed for allowing the wavelength to change continuously.
Other objects, characteristics and advantages of the present invention will be understood fully by the following description. The benefit of the present invention also will become apparent from the following description with reference to the accompanying drawings.